The present invention relates to fluid pressure control devices for vehicle braking systems, and more particularly to an improvement of a fluid pressure control device in the form of such an inertia-controlled valve unit as disclosed in U.S. Pat. No. 4,072,363 issued on Feb. 7, 1978.
A conventional inertia-controlled valve unit of this type comprises a housing provided thereon with an inlet port for connection to a master cylinder and an outlet port for connection to wheel brake cylinders and provided therein with a stepped bore in communication with the inlet and outlet ports respectively at the small and large diameter portions thereof and an inner bore provided in parallel with the stepped bore and in communication with the small and large diameter portions of the stepped bore, a stepped piston reciprocable within the stepped bore to form first and second fluid chambers respectively in communication with the inlet and outlet ports, a spring loading the piston axially in a direction toward the outlet port, and a cut-off valve assembly including a valve seat provided within the inner bore to permit the flow of fluid between the first and second chambers and an inertia-controlled valve element housed within the inner bore to co-operate with the valve seat to block the flow of fluid from the first chamber to the second chamber when the valve element is subjected to a deceleration in excess of a predetermined value. In operation of such an inertia-controlled valve unit under the normally loaded condition of the vehicle, the stepped piston acts to increase the pressure trapped in the wheel brake cylinders after closing of the cut-off valve, but, owing to the difference between the effective pressure receiving areas of the piston, the rate of increase is less than the rate of increase in the master cylinder pressure. This results in insufficient braking effect under the excessively loaded condition of the vehicle.